Nuclear fuel assemblies for powering nuclear reactors generally comprise large numbers of fuel rods that are contained in discrete fuel rod assemblies. These assemblies typically comprise a bottom end fitting or nozzle, a plurality of fuel rods extending upwardly therefrom and spaced from each other in a square or triangular pitch configuration, spacer grids situated periodically along the length of the assembly for support and orientation of the fuel rods, a plurality of control guide tubes interspersed throughout the assembly, and a top end fitting or cap. Once assembled, the fuel rod assembly can be installed within and removed from the reactor as a unit.
When the nuclear fuel rods have expended a large amount of their available energy, they are considered to be “spent,” and the fuel rod assembly is removed from the reactor and temporarily stored in an adjacent pool until they can be transported to an interim storage facility, reprocessing center, or to a permanent storage facility or repository. Even though the rods are considered to be spent, they are still highly radioactive and hazardous both to people and property.
There are a number of options available for storing and disposing of the radioactive spent fuel rods. In one such option, the fuel rod assemblies are contained within a dry storage system that can be transported offsite to another facility. In such systems, the fuel rod assemblies are typically placed, without water, within cylindrical canisters, which are then placed within transport casks.
Transportable canister-based dry spent fuel storage systems must comply with multiple federal regulatory requirements, including both storage and transport requirements. Systems that are licensed for storage must meet safety design conditions imposed by 10 CFR Part 72, while systems that are licensed for transport must meet more challenging safety design conditions that are imposed by 10 CFR Part 71 (Part 71 hereafter). These parts are the sections of the Code of Federal Regulations that stipulate the requirements that must be complied with to obtain U.S. Nuclear Regulatory Commission (NRC) certification for the storage and transport of spent fuel.
In order to achieve NRC certification under Part 71 for transport of a dry storage system for spent fuel, the storage system must be designed such that nuclear criticality cannot be achieved under normal operations and postulated accident conditions. Nuclear criticality is a condition in which the effective neutron multiplication factor of the fuel array, keff, is greater than or equal to 1.0 and a nuclear chain reaction becomes self-sustaining. According to the requirements, nuclear criticality must not be achieved even if the storage system is flooded with a neutron moderator, like water, in an optimal condition that enhances the potential for criticality. Notably, no regulatory credit is given for designing the system to ensure that water intrusion is not realistically possible.
The requirement to prevent criticality even in the presence of a neutron moderator typically forces dry storage and transport system designers to produce systems that incorporate expensive neutron absorber material in the spaces between the fuel rod assemblies. The neutron absorber material ensures that, even with a neutron moderator present, keff remains less than or equal to 0.95 and the system is not able to sustain a nuclear chain reaction. Unfortunately, such designs have relatively low fuel storage capacity and are expensive because of the need for the neutron absorber material. Furthermore, these systems are not perfectly suitable to be placed in a permanent repository because of exceedingly large dimensions, typical neutron absorber degradation uncertainties, and other canister material degradation concerns under long-term disposal conditions. The net result is that the cost per spent fuel assembly stored, transported, and disposed of is greatly increased.
From the above discussion, it can be appreciated that it would be desirable to have a transportable dry storage system and method that have higher spent fuel storage capacity and/or that remove the need for expensive neutron absorber material.